And you cannot place a tee on its back in a sanitary drain line.: : Terry -: I want to install at top of drain a 3" wye and below that a 3" combo. Down stream will be two lavarotories.: I want to vent off the top wye and reduce to 2".: My friend and part time plumber says that the vent is essentialy the soil stack and must remain 3".: He also says that I cannot lay a t on its back to drain the lavarotories into the sewer line.: I am confused can you help me.: thanks